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Why has someone who emailed me at Sussex had their message returned to them even though my address was correct?

The failure could be due to one of several reasons, but the most likely are given below.

It's possible that your mailbox is full, though this is unlikely because you would have seen warnings about this and had a chance to do something about it before it actually became full.

The Sussex email system makes a call-back, for every external incoming email, to the sender's server to verify the sender's email address.  If the remote server does not respond, and the email shows any spam-like features, then the Sussex server will reject the email.  This is called sender verification and is an anti-spam measure. The procedure is exactly the same as that used for bouncing undeliverable email and for sending autoreplies and message receipts, so if it fails, it means that the sender's server doesn't accept bounces, autoreplies or receipts either - not very useful for its users or customers!  It's possible that this is the reason for the email failing to get through to you.   The principle and method is explained in more detail in FAQ 1101.

Your correspondent should contact the technical support staff at their institution or service provider to inform them of the situation.

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This is question number 1078, which appears in the following categories:

Created by Roger Discombe on 6 March 2004 and last updated by Richard Byrom-Colburn on 31 October 2016